Formation of lubricant coating by electrolysis

ABSTRACT

A METHOD OF PROVIDING A LUBRICANT ON AN ARTICLE HAVING AN ELECTRICALL CONDUCTING SURFACE, SUCH AS A METAL ARTICLE, IN ORDER TO PROVIDE LUBRICATION DURING METAL FORMING OPERATIONS, COMPRISES MAKING THE ARTICLES TO THE COATED THE ANODE IN AN ELECTROLYTIC CELL, THE ELECTROLYTE OF THE CELL CONTAINING A SOLUBLE ORGANIC SULPHUR COMPOUND AND THEN PASSING AN ELECTRIC CURRENT THROUGH THE CELL WHEREBY A SULPHUR CONTAINING CONSTITUENT IS FORMED IN THE ELECTROLYTE FROM WHICH THE LUBRICANT COATING IS PRODUCED ON THE SURFACE OF THE ARTICLE. THE SOLUBLE ORGANIC SULPHUR COMPOUND MAY BE THIOACETAMIDE OR THIOUREA. IF DESIRED A FURTHER CONSTITUENT MAY BE PROVIDED IN THE ELECTROLYTE AND THE LUBRICANT COATING MAY BE PROVIDED AS A RESULT OF REACTION OF THE SULPHUR CONTAINING CONSTITUENT WITH THE FURTHER CONSTITUENT. THE FURTHER CONSTITUENT MAY BE AMMONIUM MOLYBDATE. THE LUBRICANT COATING MAY BE REMOVED AFTER PERFORMANCE OF THE METAL FORMING OPERATION BY PASSING THE ARTICLE THROUGH A FURTHER ELECTROLYTE THROUGH WHICH A CURRENT IS PASSED TO REMOVE THE LUBRICANT COATING.

Nov. 21, 1972 v J. P. G. FARR ETAL 3,103,449

I FORMATION OF LUBRICANT COATING BY ELECTROLYSIS Filed Oct. 29, 19703,703,449 FORMATION OF LUBRICANT COATING BY ELECTROLYSIS John PeterGeorge Farr and Geoffrey Whaley Rowe, Birmingham, England, assignors toAmerican Metal Climax Inc., New York, N.Y.

Filed Oct. 29, 1970, Ser. No. 85,107 Claims priority, application GreatBritain, Nov. 13, 1969, 55,605/ 69 Int. Cl. C23b 11/02 US. Cl. 204-56 R18 Claims ABSTRACT OF THE DISCLOSURE A method of providing a lubricanton an article having an electrically conducting surface, such as a metalarticle, in order to provide lubrication during metal formingoperations, comprises making the article to be coated the anode in anelectrolytic cell, the electrolyte of the cell containing a solubleorganic sulphur compound and then passing an electric current throughthe cell whereby a sulphur containing constituent is formed in theelectrolyte from which the lubricant coating is produced on the surfaceof the article. The soluble organic sulphur compound may bethioacetamide or thiourea. If desired a further constituent may beprovided in the electrolyte and the lubricant coating may be provided asa result of reaction of the sulphur containing constituent with thefurther constituent. The further constitutent may be ammonium molybdate.

The lubricant coating may be removed after performance of the metalforming operation by passing the article through a further electrolytethrough which a current is passed to remove the lubricant coating.

This invention relates to a method of providing a lubricant on anarticle having an electrically conducting surface, especially, but notexclusively, a metal article, such as wire, to provide lubricationduring wire drawing, or strip, to provide lubrication during rolling orpressure.

It is well known that it is desirable, and in certain cases essential,to provide lubrication during metal-working operations such as wire andtube drawing, rolling of strip, forging and deep drawing. The use of alubricant enables higher working speeds and a greater amount of metaldeformation to be obtained than is possible without a lubricant and, ofcourse, the more effective the lubricant, the higher is the speed ofworking and the greater is the amount of deformation which can beachieved.

A most effective group of lubricants in the art of metal working isthose which are commonly referred to in the art as extreme-pressurelubricants. These lubricants can provide a reactive radical such as ClorS"- which, it is thought, enters into chemical reaction with the surfaceof the metal during the metal working operation to form, for example,metal chloride or metal sulphide which acts as a lubricant. The reactiveradical in extreme-pressure lubricants is derived from an organiccompound' United States Patent l 3,793,449 Patented Nov. 21, 1972 (2)Description of the prior art By providing the reactive radical from anunreactive organic compound no such corrosive attack on the metalarticle occurs and the radical reacts with the metal only as a result ofbeing liberated from the organic compound by thermal energy createdduring the metal working operation.

Extreme-pressure lubricants have been found to be more effective inmetal working operations than simple organic lubricants especially whenhigh temperatures and pressures are created during the metal workingoperation. Extreme-pressure lubricants are especially favoured fordrawing stainless steel, titanium and nickel alloys which are otherwiseprone to transfer of metal from the workpiece to the metal working tool.

The thermal energy required to liberate the reactive radical from theorganic compound to cause the radical to react with the metal surface toprovide the lubricant is provided by plastic deformation of the metalduring working and in particular by friction and plastic deformation oflocalised protuberances on the metal article. Usually the temperaturerise of the whole articles does not provide sufiicient thermal energyfor liberation of the radical and reaction with the metal surface and sothe reaction of the radical with the metal surface is confined to theregion where localised friction and deformation, of the workpiece hastaken place and this has been demonstrated by radioactive tracertechniques.

It is therefore a characteristic feature of conventionalextreme-pressure lubrication that very close contact, and perhaps evenlocalised welding, must occur between the metal article and the metalworking tools to provide the necessary thermal energy for the abovementioned reactions to take place. Moreover, in many metal workingoperations, such as wire or tube drawing, the article moves past themetal working tools at very high speeds, for example, in the case ofwire drawing, at a speed of 1000 feet per minute through a die thecontact length of which is 0.5 inch and thus the time of contact of thewire with the die is 250 micro-seconds. Thus, the release of thereactive radical and its reaction with the metal surface must occurwithin this very short time if it is to be effective as a lubricant. Ifthe reaction takes longer than this time then the wire will have leftthe die before the reactive radical can provide a lubricant. Thus,damage to the surface of the metal article being worked and/ or to themetal working tools can occur because of the need for the very closecontact, and possibly the need for welding, to provide suflicientthermal energy to cause the reactive radical to act as a lubricant andthere is also a possibility of damage in metal working operationsperformed at a speed so fast that the metal article has moved out ofcontact with the die before the necessary reactions can occur.

SUMMARY OF THE INVENTION It is accordingly an object of the presentinvention to provide a new and improved method of providing a lubricantfilm on an article having an electrically conducting surface whereby theabove mentioned disadvantages are overcome or are reduced.

According to the invention a method of providing a lubricant coating onan article having an electrically conducting surface includes the stepsof, making the article an anode in an electrolytic cell, the electrolyteof the cell containing a soluble organic sulphur compound, and passingan electric current through the cell whereby a sulphur containingconstituent is formed in the electrolyte from which the lubricantcoating is produced on the surface of the article.

The lubricant coating may be provided as a result of electrochemicalreaction of the sulphur containing constituent with the surface of thearticle.

Alternatively, the lubricant coating may be provided as a result ofreaction of the sulphur containing constituent with a furtherconstituent of the electrolyte.

The article may be a metal article.

At least the surface of the article may be connected in an electriccircuit to act as the anode of the electrolytic cell and there being afurther electrode provided in contact with the electrolyte of the celland connected in said electric circuit to provide the cathode of thecell.

The organic sulphur compound may be thioacetamide or thiourea and theorganic sulphur compound may be in a solution of sulphuric acid in waterat a concentration lying approximately in the range 0.02 N to 0.2 N H 80Preferably, not more than 5% by weight of the organic sulphur compoundmay be present in the electrolyte.

Preferably, the organic sulphur compound is thioacetamide.

The additional constituent of the electrolyte may be a molybdenumcompound. In this case, molybdenum disulphide is the compound formed asa lubricant coating on the article.

When the additional constituent is not present in the liquid the surfaceof the article may be made of mild steel, tin, or molybdenum.

When the additional constituent is present in the electrolyte thesurface of the article may be made of mild steel, tin, stainless steel,titanium, molybdenum or a nickel alloy.

If desired, subsequent to providing a lubricant coating on a metalarticle by the method set out hereinbefore a forming operation isperformed on the article.

If desired, immediately after the forming operation the article may becontacted with an electrolyte and a relatively high alternating currentpassed through the liquid adjacent the article to remove the lubricantcoating.

The forming operation may be a drawing operation, a pressing operation,a rolling operation or a forging operation.

When the forming operation is a continuous operation such as wiredrawing or rolling the electrolyte is provided in a bath through whichthe article is passed continuously.

When the forming operation is a batch operation, such as forging, theelectrolyte is provided in a bath into which the article is placed andfrom which it is removed prior to the forming operation and, if thelubricant film is to be removed after the metal working operation thearticle is immersed in a suitable bath and then removed therefromimmediately after the metal working operation.

A still further aspect of the invention is an article produced by ametal working operation as set out above.

The method of the present invention makes it possible to provide alubricant coating on an article without relying upon thermal energycreated during the metal working operation to provide the lubricantcoating. In addition, the time taken to provide the lubricant coating isrelatively short, so that the method can be applied to continuousproduction techniques without having to slow down the rate of productionto an uneconomical level.

The method of the present invention therefore has the considerableadvantage that the formation of the lubricant coating can be closelycontrolled by means of the applied potential and current densityindependently of the speed of the metal working operation or upon closeproximity or welding between the metal article and the metal workingtool thereby avoiding the problem mentioned hereinbefore of possibledamage to the metal article or metal working tool as a result of suchclose contact and possible welding before it is possible for theextreme-pressure lubricants used hereto to provide a lubricating action.

In addition, the amount of lubricant present can be simply and easilycontrolled merely by adjusting the potential and current density and/oradjusting the period 4 of time for which the liquid is in contact withthe metal article. For example, in the case of a continuous metalworking operation such as wire drawing the length of the bath throughwhich the wire is passed immediately prior to drawing may be chosen sothat it is of an appropriate length for the intended speed of drawing.In the case of a batch process such as forging or deep drawing then thetime for which the article is immersed in the bath is adjusted asnecessary to give the desired amount of lubricant coating.

Moreover in operation such as deep drawing it is sometimes desirable toproduce lubricant films at certain 10- calised positions so as toencourage differential metal flow during drawing and this can beachieved with the method of the present invention, for example, bypositioning a further electrode in relatively close proximity to themetal article in the regions where it is desired to produce the locallubricant film so that the majority of the lubricant coating is formedat these regions and relatively little or no lubricant coating isprovided at other regions.

With some extreme-pressure lubricants used hitherto the organic compoundof the lubricant containing the reactive radicals can decomposeespecially in the presence of moisture and this leads to corrosion ofthe metal article and it has therefore been necessary to remove theextremepressure lubricants and their decomposition products after themetal working operation. This has usually been performed by two separateoperations using organic solvents and inorganic fluxes. In the method ofthe present invention the coating can be removed immediately after themetal working operation by contacting the metal article with a liquid ofthe same composition as the carrier liquid with which the liquid iscontacted before the metal working operation and establishing analternating potential difference in the liquid adjacent the metalarticle to disrupt the lubricant film and so restore the metal to itsoriginal condition.

The apparatus used in the removal operation is similar to that used inthe appropriate process for the lubricant coating forming operation.

In those cases where the method is performed with the electrolyte notcontaining said additional constituent the lubricant coating achieved isa compound of the metal of which the article is made. For example, ifthe metal is a mild steel then the lubricant coating will be ironsulphide.

If however said further constituent is provided in the liquid then thelubricant formed is not dependent upon the metal of which the article ismade. For example, if the further constituent is a molybdenum compoundthen molybdenum disulphide will be formed in the electrolyte and willdeposit on the surface of the article to form a surface layer ofmolybdenum disulphide irrespective of the metal of which the article ismade. Molybdenum disulphide is recognised to give better lubricationthan iron sulphide for example.

The precise mechanism which causes the molybdenum disulphide to depositon the metal wire and the nature of the bonding of the molybdenumdisulphide to the metal wire is not fully understood but the molybdenumdisulphide is relatively strongly bonded onto the metal article althoughapparently not so strongly as is the lubricant film formed as a resultof reaction with the metal of the wire.

It is therefore possible with the method of the present invention toprovide, relatively strongly bonded to the metal article, a lubricantsuch as molybdenum disulphide (which is well known as an outstandinglygood lubricant) irrespective of the nature of the metal of which thearticle is made whereas with conventional extreme-pressure lubricantsthis has not been possible and only compounds of the metal of which thearticle is made have been possible. Five examples of the methodaccording to the present invention will now be described by way ofexample.

DESCRIPTION OF THE PREFERRED EMBODIMENT Example 1 Annealed 1.2 mm. dia.molybdenum wire was cold drawn to 8% R.A. through a tungsten carbide dieat a speed of 9 ft./min. With no lubrication the wire sufferedconsiderable surface damage and the drawing load was in excess of 150lb. With a liberal application of a graphite-bearing oil normally usedfor drawing M the load was reduced to 90-110 1b., the wire this timehaving a satisfactory surface finish. A length of the wire was made theanode in a tank containing N/S H2SO +l0 gm./l. thioacetamide+10 gm./l.ammonium molybdate and a current of 100 ma./cm. was passed for 10seconds from a 6-volt battery. When this wire was drawn a 90- 120 lb.load was recorded, and scanning electron micrographs showed the surfacefinish of this wire to be slightly superior to that of the oil-drawnwire.

Example 2 Annealed mild steel wire of 1.0 mm. dia. was cold drawn to 12%R.A. through a tungsten carbide die at 6 ft./min. With no lubricantpresent the wire surface was heavily scored and the drawing load was 105lb. With a conventional chlorinated extreme-pressure lubricant the loadwas 93 lb. and the wire had a satisfactory surface finish. When ametallic soap lubricant was used the surface was also satisfactory butthe load was reduced to 75 lb. With a lubricant formed electrolyticallyas in Example l a load of 70 lb. was recorded and again a good surfacefinish was achieved.

Example 3 Annealed 18/8 stainless steel wire of 1.0 mm. dia. was colddrawn to 25% R.A. through a tungsten carbide die at 6 ft./min. Without alubricant there was considerable metallic pick-up and the drawing loadwas 197 lb. With a metallic soap lubricant the surface was muchimproved, the drawing load this time being 147 lb. With a lubricantelectrolytically formed as in Example 1 the surface finish was againsatisfactory and the drawing load was 145 1b.

Example 4 Annealed mild steel wire was drawn to 5% R.A. with nolubricant. There was considerable pick-up and a drawing force of 70 lb.was required. Another sample of the wire was made the anode in a cellcontaining N/ H SO +10 gm./litre thioacetamide and a current of 300ma./cm. was passed for 10 secs. The force needed to draw this wire was'63 lb. and the surface finish was satisfactory.

Example 5 Annealed mild steel wire was drawn to 5% R.A. with nolubricant. There was considerable pick-up and a drawing force of 70 lb.was required. Another sample of the wire was made the anode in a tankcontaining N/10 H SO +10 gm./litre thiourea and a current of 300 ma./cm. was passed for 10 sec. The force needed to draw this wire was 63 lb.and the surface finish again was satisfactory.

BRIEF DESCRIPTION OF THE DRAWINGS The apparatus used in the examplesdescribed hereinbefore is illustrated in the accompanying drawing whichis a diagrammatic representation of the apparatus used.

Referring now to the drawing, the apparatus comprises a wire drawing die10 which can be made of tungsten carbide. The metal working die 10 ismounted in a die block 11 provided with a load cell 12 to enable thedrawing load to be measured. The die block 11 is provided on aconventional draw bench, not shown, and the wire 14 under test is drawnthrough the die 10 by a conventional capstan 15 provided on the drawbench.

0n the input side of the die 10 the wire 14 is passed through anelectrolytic bath 16 provided with suitable seals 17 of neoprene rubberand which contains the solution described in the examples. Counterelectrodes 18 are also mounted in the bath 16 above and spaced from thewire 14 and the counter electrode 18 is connected through an ammeter 19to a battery 20 whilst the wire 14 is connected in electric circuit withthe positive terminal of the battery 20 through a variable resistance21. The connection to the moving wire 14 is made through contactsprovided at the die 10.

When it is desired to remove the lubricant immediately after drawing abath 22 similar to the bath 16 is provided on the output side of the die10 so that the wire 14 is drawn through both baths by the capstan 15 ofthe draw bench. In this second bath the wire is connected in circuitwith an alternating current source 23.

When the method is performed in a rolling operation on strip or sheet abath of suitable width is provided and is provided with suitable sealingmeans to prevent leakage of the solution as the sheet or strip entersand leaves the bath.

When the method of the present invention is performed on batchoperations, such as forging or deep drawing, a bath of suitabledimensions, open at the top, is provided and the article to be worked islowered into the bath for an appropriate period of time to form acoating of appropriate thickness or to remove the coating asappropriate.

If a lubricant coating is required only on certain regions of theworkpiece the counter electrode is made of appropriate shape so that thecoating is deposited only in the regions adjacent the counter electrode.

We claim:

1. A method of providing a lubricant coating on an article having anelectrically conducting surface including the steps of, providing anelectrolytic cell including a cathode, making the article an anode insaid electrolytic cell, providing an aqueous electrolyte for the cellcontaining a soluble organic sulphur compound, and passing an electriccurrent through the cell whereby a sulphur containing constituent isformed in the electrolyte from which the lubricant coating is producedon the surface of the article.

2. A method according to claim 1 wherein the sulphur containingconstituent electrochemically reacts with the surface of the article toprovide the lubricant coating.

3. A method according to claim 1 including providing a solublemolybdenum compound as a further constituent of said electrolyte whereinthe sulphur containing constituent reacts with said molybdenum compoundof the electrolyte to provide the lubricant coating.

4. A method according to claim 3, wherein the molybdenum compound isammonium molybdate.

5. A method according to claim 1, wherein the article is a metalarticle.

6. A method according to claim 1, wherein the organic sulphur compoundis thioacetamide.

7. A method according to claim 1 wherein the organic sulphur compound isthiourea.

8. A method according to claim 1, wherein the organic sulphur compoundis in a solution of sulphuric acid in water at a concentration lyingapproximately in the range 0.02 N to 0.2 N H 50 9. A method according toclaim 1, wherein not more than 5% by weight of the organic sulphurcompound is present in the electrolyte.

10. A method according to claim 1, wherein subsequent to providing thelubricant coating a forming operation is performed on the article.

11. A method according to claim 10, wherein the article is a metalarticle and the forming operation is a drawing operation, a pressingoperation, a rolling operation, or a forging operation.

12. A method according to claim 10, wherein the forming operation isperformed immediately subsequent to providing the lubricant coating.

13. A method according to claim 10, wherein after the forming operationthe metal article is contacted with a further electrolyte and analternating electrical current is applied in the liquid adjacent thearticle to remove the lubricant coating.

14. A method according to claim 10 wherein the forming operation is acontinuous operation and the electrolyte is provided in a bath throughwhich the metal article is continuously passed.

15. A method according to claim 10 wherein the forming operation is abatch operation and the electrolyte is provided in a bath into which thearticle is placed and from which the article is removed prior to theforming operation.

16. A method according to claim 13 wherein the forming operation is acontinuous operation and the further electrolyte, with which the articleis contacted after the forming operation, is provided in a bath throughwhich the article is continuously passed.

17. A method according to claim 13 wherein the forming operation is abatch operation and the further electrolyte with which the article iscontacted after the forming operation is contained in a bath into whichthe article is placed and from which it is removed subsequent to theforming operation.

18. A method according to claim 1, wherein the cathode is positioned inoperative relation to the metal article only in those regions where itis desired to produce a lubricant film so that the local thickness ofthe lubricant film produced on the article may be adjusted.

References Cited UNITED STATES PATENTS 3,591,472 7/ 1971 Amsallem 20456R 3,438,789 4/1969 Weiss et al 204-56 R 2,512,141 6/1950 Ma et a1.204--56 R FOREIGN PATENTS 201,345 8/1967 U.S.S.R 204-92 JOHN H. MACK,Primary Examiner R. L. ANDREWS, Assistant Examiner

